The present invention is related to a general method and apparatus for optically measuring the position of a single or multiple mechanical components.
There are numerous examples of using the deflection from a single optical beam for positional measurements, the optical lever detection system commonly used in atomic force microscopes (AFMs) being perhaps the most notable. One such optical lever system is illustrated in FIG. 1. In this system a light beam 2, preferably formed by a laser 1 (including a superluminescent laser or diode) with sufficient intensity and lack of pointing or other noise, is directed through a collimation lens or lens assembly 3 and a focusing lens or lens assembly 5 and onto a mirror 6 which directs the focused light beam 7 onto a particular spot on a cantilever 8 in the same reference frame as the optical lever system. The reflected beam 9 is then collected by detection optics, which often include an adjustable mirror and a translation stage for providing an offset to the beam position (not shown), and made to illuminate a position sensitive detector 10 (PSD). As the cantilever moves in response to various forces, the position of the reflected spot on the PSD changes, causing a change in the output. It will be noted that the optical axis of the system 27 coincides with the axis of the light beam as it propagates through the system.
Another prior art AFM optical lever system, in which the cantilever and the optical lever system are in different reference frames, attempts to track the position of the cantilever as it is scanned over a surface. There are a number of schemes to accomplish this. The most successful, based on a tracking lens that moves with the piezo tube scanning the cantilever, is described in U.S. Pat. No. 6,032,518.
There has been a great deal of work on optimizing the sensitivity of AFM optical lever detection systems. All of the optimization techniques can be implemented using the invention disclosed herein and most are easier to implement using the invention.
The employment of two- or multiple-beam systems in positional measurement instruments provides significant advantages. In the case of AFMs and other scanning probe instruments, a second beam can provide a reference for more accurate positional measurements. A second or multiple beams can also allow more than one cantilever probe to be used in imaging. In the case of micromechanical sensors, a second or multiple optical beams can be used to provide a baseline reference signal for comparison with the active sensor element to compensate for thermal drift or other effects. Two or more beams also make it possible to simultaneously observe more sensors, thereby increasing throughput. For optical profilometers, multiple beams offer the possibility of increasing throughput or simultaneous monitoring of several positions.
There are a number of multiple beam systems in the literature. To date, these systems rely upon two or more separate light sources focused onto different locations. This complexity has limited the use of multiple beam sensor arrays in any number of commercial applications including high throughput scanning probe microscopes or micromechanical sensors containing numerous sensing elements and control levers for background measurements.
Diffractive Optical Elements (DOEs) provide a flexible and powerful means for splitting the beam from a single source of light into multiple beams and varying the intensity and shape of each beam. Using a DOE it is possible to illuminate an array of cantilevers or other mechanical structures using only one light source. The spacing between the focused spots and the spot geometry can be controlled. The multiple beams can also be shaped to vary the sensitivity of the measurement and the beams can be steered either individually or as a group.
Shaping the spot has important consequences for cantilever based force measurements; it is possible to minimize lost optical power and therefore spurious interference effects as well as optimizing the optical lever sensitivity with a correctly chosen beam shape. By changing the beam shape as well as the position, it is possible to vary the optical lever sensitivity. It is also possible to vary the dc offset of the detector. DOEs make a continuum of beam shapes available to the experimenter. For optical profiling applications, changing the beam shape allows the resolution of the profilometer to be tuned to the application.
Finally DOEs may be used to modulate the intensity of a single or multiple beams, allowing a variety of other measurements to be made. One example is that this modulation can be used to allow synchronous detection of the position or angle of the sensing element. In the case of sensitive transducers, it is also possible to use the modulated optical energy to actuate the illuminated object, either through light pressure or a number of thermal effects.
There are a variety of commercial DOEs available off the shelf. Numerous manufacturers can fabricate OEM components to a variety of specifications. If active DOEs are used (such as phase shifting liquid crystals or phase shifting reflective mirrors) the beam shape can be dynamically changed as different cantilevers are used. A further advantage of active DOEs is that not only beam shape but also the beam position can be controlled. This allows the beam position to be chosen without any moving mechanical parts. It also makes it possible to change the relative position of the cantilever and detector during the experiment while maintaining the spot focused on the lever. This ability to track the cantilever position means that a variety of beam-tracking AFMs can be realized that do not depend on complicated mechanical apparatus or on heavy optical systems that are scanned along with the moving sensor.
One challenge of a multiple sensor system used for chemical, biological or other sensing applications is separating the beams once they have reflected off the sensors. This can be accomplished with a suitable arrangement of lenses that are used to collect the light and separate it allowing the use of multiple PSDs. As mentioned above, programmable DOEs allow the possibility of modulating the intensity of individual beams in an array, allowing it to be unambiguously identified by a PSD even in the presence of other beams or other background noise. Again, as mentioned above, all of this is accomplished without the use of any moving parts. These arrays can also be translated by changing the DOE diffraction grating or hologram to account for changes in the cantilever position, either intentional (such as a positional change associated with scanning) or incidental (such as thermal drift) during the course of an experiment or measurement.
It has been pointed out that optical beams either through photon momentum changes, thermal effects or other means can cause mechanical changes in micromechanical components. DOE based sensors are compatible with a positional measurement being made with one beam while another is used to effect mechanical changes. Again, this can be accomplished with one light source if the experimenter wishes. Examples include exciting oscillations in a cantilever by sinusoidally varying the optical intensity and canceling the effects of thermal noise to enable low noise force measurements. As above, it is also possible to do this with an arbitrarily shaped array of a plurality of micromechanical components. Also as above, the beams can be translated either individually or as a unit during the course of the experiment by appropriate changes of the DOE diffraction grating or hologram.
In the case of translating beam spots, the appropriate diffraction or hologram could be calculated ahead of time, stored and simply played back to the active DOE when necessary.